A. Field
This disclosure relates generally to cellular wireless communication and more particularly to methods for efficiently allocating soft hand-off resources, including channel elements and Walsh codes, in one or more base transceiver stations. The disclosure further relates to methods for intentionally removing a sector from a mobile station's active set (and thereby freeing up soft hand-off resources) where such elimination does not have a significant adverse impact on communications between a mobile station and other sectors in the mobile station's active set.
B. Related Art
Cellular wireless communication is a technology which allows roaming mobile stations such as cellular telephones to make and receive telephone calls and exchange data with network entities. In the United States, a spread spectrum technology known as CDMA (Code Division Multiple Access) has been deployed. The CDMA technology is described in publicly available standards documents, including IS 95-A, and in standard reference textbooks.
The infrastructure for CDMA includes a network of cells having base transceiver stations, each having an antenna for communicating with a mobile wireless device via radio frequency (RF) communication. The cells are further divided into sectors. CDMA technology includes a feature providing for handoff for cellular wireless communication devices, i.e., facilitating transfer of a device from one wireless coverage area (e.g., cell sector) to another during a call. “Hard handoff” involves switching the device out of a first sector before switching the device to a new sector (i.e., break-before-make), which could result in dropped calls. “Soft handoff”, which is the more preferred approach, involves switching the device to a new sector before switching the device out of the old sector (i.e., make-before-break), which allows for more seamless transfer of a communication session from one sector to another.
In a CDMA system operating according to the “1xRTT” (“1x”) protocol, a mobile station maintains in its data storage a listing of sectors called its “active set,” which may be provided by one or more base transceiver stations in the mobile station's area. Each sector has a respective Pseudo-Noise (PN) offset, and an assigned Walsh code, which are used as a basis to uniquely modulate signals transmitted in that sector. When the mobile station is engaged in a call (or other bearer communication), the mobile station actually listens to all of its active set sectors at once. Several alternative methods of combining or using the all the active set sectors are known. In one example, a mobile station combines the signals over time by picking the best one on a frame-by-frame basis. In another example, the mobile station picks the earliest arriving frame, provided it has a good cyclical redundancy check (CRC).
Communication between the mobile station and a sector in a base transceiver station is handled on the base transceiver side by a device known as a channel element. In some implementations, for a particular set of N sectors in an active set, N channel elements are employed. In other implementations, fewer than N channel elements are needed because they are shared across more than one sector. For a given mobile station, the channel elements need not be all located in a particular base transceiver station. For example, two channel elements acting as sectors in an active set could be located in one base transceiver station and another channel element could be could be located in a separate base transceiver station.
In addition to the “active set”, a mobile station also maintains a list of “neighbor set” sectors, which are adjacent sectors to which the mobile station can theoretically hand off. In operation, the mobile station regularly receives and measures the strength of pilot signals on each of its active set sectors and each of its neighbor set sectors. If the received pilot signal power in a neighbor set sector exceeds a predefined threshold (called “TADD”), the mobile station then sends a Pilot Strength Management Message (PSMM) over the air to its serving base station controller (BSC), proposing a new active set for the mobile station to use. The new proposed set would be the old set, modified to add the neighbor sector and possibly delete an active sector. If the active set is full, a comparison between the requested neighbor and the lowest power active set member is conducted to see if a swap should occur. If the BSC approves of a change to the active set, the base transceiver station then sends a Handoff Direction Message (HDM) to the mobile station, directing the mobile station to begin using the new active set in place of the old active set. This is a soft handoff process, because the mobile station still continues to communicate on one or more sectors on which it was communicating before the handoff occurred.
As mobile cellular devices are typically battery operated devices with finite battery capacity, power management features are included in CDMA technology which are designed to minimize power consumption of a wireless device when communicating with a base transceiver station. However, transmit power levels from the mobile station have to be high enough that the base transceiver station can communicate with the mobile station without excessive error rates. Consequently, in a CDMA system a mobile station and base station regularly engage cooperatively in a power control process to control the power of bearer transmissions over the air interface.
Generally speaking, on the reverse link of the air interface (i.e., from the mobile station to the base transceiver station), the base transceiver station monitors the power of signals the base transceiver station receives from the mobile station and may direct the mobile station to transmit at a higher power level if the received power level is not high enough. Further, the base station will monitor the frame error rate (FER) of the signals that it receives from the mobile station and may direct the mobile station to transmit at a higher power level if the FER is too high. This power control is carried out in an “open loop” or a “closed loop” depending on the status of the call. The power control process is known in the art and further details are therefore omitted from the present discussion.
On the forward link, a somewhat similar process occurs. The mobile station will monitor the FER of signals that the mobile station receives from the base transceiver station, and the mobile station will report the FER to the base transceiver station. If the base transceiver station then determines that the mobile station's detected FER is too high, the base transceiver station will then increase its transmit power to the mobile station.
As noted above, a cellular base transceiver station contains a number of “channel elements,” which are typically circuit cards that are arranged to support air interface communication. When a base transceiver station and mobile station are in communication in a particular cell sector, a channel element is dedicated for that communication. Thus, if a mobile station communicates at once on three active set sectors, up to three channel elements would be used for the communication, depending on whether channel elements were shared. Channel elements are a finite (and expensive) resource, and need to be managed in an efficient manner. Likewise, there are a finite number of Walsh codes that are available for modulating signals on sectors in a base transceiver station and there is a need in the art to more efficiently handling these soft hand-off resources as well.
Further background information on CDMA is set forth in the patent literature (see e.g., U.S. Pat. Nos. 6,317,453 to Chang and et al., 6,058,136 Gansesh), the relevant standards documents, and in web-based tutorials and downloadable resources, such as “Course 132 Technical Introduction to CDMA, IS-95 CDMA and a few details of CDMA 2000 1X”, Scott Baxter (2003), available at www.howcdmaworks.com.